Factorial validity and norm data comparison of the Short Form 12 in patients with inflammatory-rheumatic disease

Measuring quality of life is increasingly considered as an outcome criterion in clinical studies. In order to assess quality of life, disease specific as well as generic instruments are used. Generic instruments make it possible to compare outcomes among different indications, but for this purpose a test of the factorial validity of the method in each indication is necessary. In this study, the generic short form 12 was administered to a sample survey of 545 patients with inflammatory-rheumatic disease. Patients data from three scientific rehabilitation research projects were pooled for the analysis. First the structure was tested using confirmatory structural equation modeling. In a second step age and gender specific values were calculated and compared to norm data from the German National Health Survey 1998. The questionnaire's structure is acceptable and comparable to international results. Confirmatory analyses support a model allowing covariations of error terms between items of the same subdimensions. The two latent dimension are highly correlated. Crossloadings of items from different subdimension does not improve the model fit significantly. Comparisons with the German norm data shows that the patients are impaired within their physical and mental dimension of subjective health. Higher impairment is evident especially on the physical sumscale. Women show higher impairment in both scales. However no effects of age can be detected. The SF-12 could be used with trust in the study of patients with inflammatory-rheumatic diseases. But physical and mental health can not be seen as independently.